Fix NewBook write mechanism, bundle push extension, remove vestigial hotel_id

- setBookingCustomFields: use bookings_update's own custom_fields array
  (confirmed from the real legacy plugin code) instead of
  instance_custom_fields_set, which was only ever proven for a different,
  unrelated plugin
- online_checkin_complete is now a configurable per-hotel custom field
  (online_checkin_complete_custom_field_id setting), not a bogus top-level
  bookings_update field NewBook would silently ignore
- Fix field-name mismatches vs a real live NewBook booking response:
  booking_arrival/departure/length, guest address (street/city/state/
  postcode/country, not a single field), phone/mobile (in contact_details,
  not top-level), and pickPrimaryGuest now checks primary_client (was
  checking fields that don't exist in the real API)
- Bundle the standalone push extension into backend/extension-src/, add
  GET /api/extension/download (zips on request via archiver) and a
  download button on the Settings page
- Remove newbook_hotel_id — vestigial from the legacy system's multi-hotel
  routing model, never actually reached NewBook's API, this stack is
  single-hotel-per-deployment so there's nothing to route between

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/extension-src/popup/popup.css

@@ -0,0 +1,47 @@
+body {
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
+  width: 300px;
+  margin: 0;
+  padding: 12px 14px;
+  color: #1e293b;
+  background: #f4f5f7;
+}
+.header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: 10px;
+}
+.title { font-weight: 700; font-size: 14px; color: #1a1a2e; }
+.nb-status { font-size: 11px; padding: 2px 8px; border-radius: 999px; }
+.nb-status.ok { background: #dcfce7; color: #166534; }
+.nb-status.no { background: #fee2e2; color: #991b1b; }
+.muted { color: #64748b; font-size: 13px; }
+.booking { font-size: 13px; margin-bottom: 10px; }
+.booking strong { color: #1a1a2e; }
+select {
+  width: 100%;
+  box-sizing: border-box;
+  padding: 8px;
+  border-radius: 8px;
+  border: 1px solid #e4e8ee;
+  margin-bottom: 10px;
+  font-size: 13px;
+}
+button.primary {
+  width: 100%;
+  padding: 10px;
+  border: none;
+  border-radius: 8px;
+  background: #c9a84c;
+  color: #1a1a2e;
+  font-weight: 700;
+  font-size: 13px;
+  cursor: pointer;
+}
+button.primary:disabled { opacity: 0.5; cursor: default; }
+.msg { margin-top: 10px; font-size: 12px; }
+.msg.error { color: #dc2626; }
+.msg.ok { color: #16a34a; }
+.msg.warn { color: #b45309; }
+a.settings-link { display: block; margin-top: 12px; font-size: 12px; color: #7b4f00; }